Environmentally sequestered spent fuel pool

ABSTRACT

An environmentally sequestered nuclear spent fuel pool in one embodiment includes sidewalls and a base slab that confine a water impoundment. The pool includes fuel racks containing spent fuel assemblies which heat the water via radioactive decay. A dual liner system enclosing the pool forms an impervious barrier providing redundant provisions for preventing leakage of contaminated pool water into the environment. An interstitial space is formed between the liners which may be maintained at sub-atmospheric pressures by a vacuum pump system that evacuates the space. By maintaining the pressure in the space at a negative pressure with corresponding boiling point less than the temperature of the pool water, any leakage through the inner-most liner into the interstitial space will vaporize and be extracted via the pump for signaling a potential leak in the liner system.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 14/877,217, filed Oct. 7, 2015, which claims the benefit ofpriority to U.S. Provisional Application No. 62/061,089, filed Oct. 7,2014, the entireties of which are incorporated herein by reference.

BACKGROUND

The present invention generally relates to storage of nuclear fuelassemblies, and more particularly to an improved spent fuel pool for wetstorage of such fuel assemblies.

A spent fuel pool (sometimes, two or more) is an integral part of everynuclear power plant. At certain sites, standalone wet storage facilitieshave also been built to provide additional storage capacity for theexcess fuel discharged by the reactors. An autonomous wet storagefacility that serves one or more reactor units is sometimes referred toby the acronym AFR meaning “Away-from-Reactor.” While most countrieshave added to their in-plant used fuel storage capacity by building drystorage facilities, the French nuclear program has been the most notableuser of AFR storage.

As its name implies, the spent fuel pool (SFP) stores the fuelirradiated in the plant's reactor in a deep pool of water. The pool istypically 40 feet deep with upright Fuel Racks positioned on its bottomslab. Under normal storage conditions, there is at least 25 feet ofwater cover on top of the fuel to ensure that the dose at the pool decklevel is acceptably low for the plant workers. Fuel pools at most (butnot all) nuclear plants are at grade level, which is desirable from thestandpoint of structural capacity of the reinforced concrete structurethat forms the deep pond of water. To ensure that the pool's water doesnot seep out through the voids and discontinuities in the pool slab orwalls, fuel pools in nuclear plants built since the 1970s have alwaysbeen lined with a thin single-layer stainless steel liner (typically inthe range of 3/16 inch to 5/16 inch thick). The liner is made up ofsheets of stainless steel (typically ASTM 240-304 or 304L) seam weldedalong their contiguous edges to form an impervious barrier between thepool's water and the undergirding concrete. In most cases, the weldedliner seams are monitored for their integrity by locating a leak chasechannel underneath them (see, e.g. FIG. 1). The leak chase channels'detection ability, however, is limited to welded regions only; the basemetal area of the liner beyond the seams remains un-surveilled.

The liners have generally served reliably at most nuclear plants, butisolated cases of water seepage of pool water have been reported.Because the pool's water bears radioactive contaminants (most of itcarried by the crud deposited on the fuel during its “burn” in thereactor), leaching out of the pool water to the plant's substrate, andpossibly to the underground water, is evidently inimical to publichealth and safety. To reduce the probability of pool water reaching theground water, the local environment and hence some AFR pools haveadopted the pool-in-pool design wherein the fuel pool is enclosed by asecondary outer pool filled with clean water. In the dual-pool design,any leakage of water from the contaminated pool will occur into theouter pool, which serves as the barrier against ground watercontamination. The dual pool design, however, has several unattractiveaspects, viz.: (1) the structural capacity of the storage system isadversely affected by two reinforced concrete containers separated fromeach other except for springs and dampers that secure their spacing; (2)there is a possibility that the outer pool may leak along with the innerpool, defeating both barriers and allowing for contaminated water toreach the external environment; and (3) the dual-pool designsignificantly increases the cost of the storage system.

Prompted by the deficiencies in the present designs, a novel design of aspent nuclear fuel pool that would guarantee complete confinement ofpool's water and monitoring of the entire liner structure includingseams and base metal areas is desirable.

SUMMARY

The present invention provides an environmentally sequestered spent fuelpool system having a dual impervious liner system and leakdetection/evacuation system configured to collect and identify leakagein the interstitial space formed between the liners. The internal cavityof the pool has not one but two liners layered on top of each other,each providing an independent barrier to the out-migration (emigration)of pool water. Each liner encompasses the entire extent of the wateroccupied space and further extends above the pool's “high water level.”The top of the pool may be equipped with a thick embedment plate(preferably 2 inches thick minimum in one non-limiting embodiment) thatcircumscribes the perimeter of the pool cavity at its top extremityalong the operating deck of the pool. Each liner may be independentlywelded to the top embedment plate. The top embedment plate features atleast one telltale hole, which provides direct communication with theinterstitial space between the two liner layers. In one implementation,a vapor extraction system comprising a vacuum pump downstream of aone-way valve is used to draw down the pressure in the inter-liner spacethrough the telltale hole to a relatively high state of vacuum. Theabsolute pressure in the inter-liner space (“set pressure”) preferablyshould be such that the pool's bulk water temperature is above theboiling temperature of water at the set pressure as further describedherein.

In one embodiment, an environmentally sequestered nuclear spent fuelpool system includes: a base slab; a plurality of vertical sidewallsextending upwards from and adjoining the base slab, the sidewallsforming a perimeter; a cavity collectively defined by the sidewalls andbase slab that holds pool water; a pool liner system comprising an outerliner adjacent the sidewalls, an inner liner adjacent the outer linerand wetted by the pool water, and an interstitial space formed betweenthe liners; a top embedment plate circumscribing the perimeter of thepool at a top surface of the sidewalls adjoining the cavity; and theinner and outer sidewalls having top terminal ends sealably attached tothe embedment plate.

In another embodiment, an environmentally sequestered nuclear spent fuelpool with leakage detection system includes: a base slab; a plurality ofvertical sidewalls extending upwards from and adjoining the base slab,the sidewalls forming a perimeter; a cavity collectively defined by thesidewalls and base slab that holds pool water; at least one fuel storagerack disposed in the cavity that holds a nuclear spent fuel assemblycontaining nuclear fuel rods that heat the pool water; a pool linersystem comprising an outer liner adjacent the sidewalls and base slab,an inner liner adjacent the outer liner and wetted by the pool water,and an interstitial space formed between the liners; a top embedmentplate circumscribing the perimeter of the pool, the embedment plateembedded in the sidewalls adjoining the cavity; the inner and outerliners attached to the top embedment plate; a flow plenum formed alongthe sidewalls that is in fluid communication with the interstitialspace; and a vacuum pump fluidly coupled to the flow plenum, the vacuumpump operable to evacuate the interstitial space to a negative setpressure below atmospheric pressure.

A method for detecting leakage from a nuclear spent fuel pool isprovided. The method includes: providing a spent fuel pool comprising aplurality of sidewalls, a base slab, a cavity containing cooling water,and a liner system disposed in the cavity including an outer liner, aninner liner, and an interstitial space between the liner; placing a fuelstorage rack in the pool; inserting at least one nuclear fuel assemblyinto the storage rack, the fuel assembly including a plurality of spentnuclear fuel rods; heating the cooling water in the pool to a firsttemperature from decay heat generated by the spent nuclear fuel rods;drawing a vacuum in the interstitial space with a vacuum pump to anegative pressure having a corresponding boiling point temperature lessthan the first temperature; collecting cooling water leaking from thepool through the liner system in the interstitial space; converting theleaking cooling water into vapor via boiling; and extracting the vaporfrom the interstitial space using the vacuum pump; wherein the presenceof vapor in the interstitial space allows detection of a liner breach.The method may further include discharging the vapor extracted by thevacuum pump through a charcoal filter to remove contaminants. The methodmay further include: monitoring a pressure in the interstitial space;detecting a first pressure in the interstitial space prior to collectingcooling water leaking from the pool through the liner system in theinterstitial space; and detecting a second pressure higher than thefirst pressure after collecting cooling water leaking from the poolthrough the liner system in the interstitial space; wherein the secondpressure is associated with a cooling water leakage condition.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the exemplary embodiments will be described withreference to the following drawings where like elements are labeledsimilarly, and in which:

FIG. 1 is a cross sectional diagram of a known approach used to monitorthe integrity of weld seams for leakage in a single spent fuel poolliner system;

FIG. 2 is a side cross-sectional view of an environmentally sequesterednuclear spent fuel pool having a dual liner and leakage collection andmonitoring system according to the present disclosure;

FIG. 3 is a top plan view of the fuel pool with liner and leakagecollection/monitoring system of FIG. 2;

FIG. 4 is a detail taken from FIG. 2 showing a bottom joint of the linersystem at the intersection of liners from the sidewalls and base slab ofthe fuel pool;

FIG. 5 is a detail taken from FIG. 2 showing a top joint of the linersystem at the terminal top ends of the sidewall liners;

FIG. 6 is a perspective view of an example nuclear fuel assemblycontaining spent nuclear fuel rods; and

FIG. 7 is a schematic diagram of a vacuum leakage collection andmonitoring system according to the present disclosure.

All drawings are schematic and not necessarily to scale. Parts shownand/or given a reference numerical designation in one figure may beconsidered to be the same parts where they appear in other figureswithout a numerical designation for brevity unless specifically labeledwith a different part number and described herein. References herein toa figure number (e.g. FIG. 1) shall be construed to be a reference toall subpart figures in the group (e.g. FIGS. 1A, 1B, etc.) unlessotherwise indicated.

DETAILED DESCRIPTION

The features and benefits of the invention are illustrated and describedherein by reference to exemplary embodiments. This description ofexemplary embodiments is intended to be read in connection with theaccompanying drawings, which are to be considered part of the entirewritten description. Accordingly, the disclosure expressly should not belimited to such exemplary embodiments illustrating some possiblenon-limiting combination of features that may exist alone or in othercombinations of features.

In the description of embodiments disclosed herein, any reference todirection or orientation is merely intended for convenience ofdescription and is not intended in any way to limit the scope of thepresent invention. Relative terms such as “lower,” “upper,”“horizontal,” “vertical,”, “above,” “below,” “up,” “down,” “top” and“bottom” as well as derivative thereof (e.g., “horizontally,”“downwardly,” “upwardly,” etc.) should be construed to refer to theorientation as then described or as shown in the drawing underdiscussion. These relative terms are for convenience of description onlyand do not require that the apparatus be constructed or operated in aparticular orientation. Terms such as “attached,” “affixed,”“connected,” “coupled,” “interconnected,” and similar refer to arelationship wherein structures are secured or attached to one anothereither directly or indirectly through intervening structures, as well asboth movable or rigid attachments or relationships, unless expresslydescribed otherwise.

Referring to FIGS. 2-6, an environmentally sequestered spent fuel poolsystem includes a spent fuel pool 40 comprising a plurality of verticalsidewalls 41 rising upwards from an adjoining substantially horizontalbase wall or slab 42 (recognizing that some slope may intentionally beprovided in the upper surface of the bottom wall for drainage toward alow point if the pool is to be emptied and rinsed/decontaminated at sometime and due to installation tolerances). The base slab 42 and sidewalls41 may be formed of reinforced concrete in one non-limiting embodiment.The fuel pool base slab 42 may be formed in and rest on the soilsub-grade 26 the top surface of which defines grade G. In thisembodiment illustrated in the present application, the sidewalls areelevated above grade. In other possible embodiments contemplated, thebase slab 42 and sidewalls 41 may alternatively be buried in sub-grade26 which surrounds the outer surfaces of the sidewalls. Eitherarrangement may be used and does not limit of the invention.

In one embodiment, the spent fuel pool 40 may have a rectilinear shapein top plan view. Four sidewalls 41 may be provided in which the poolhas an elongated rectangular shape (in top plan view) with two longeropposing sidewalls and two shorter opposing sidewalls (e.g. end walls).Other configurations of the fuel pool 40 are possible such as squareshapes, other polygonal shapes, and non-polygonal shapes.

The sidewalls 41 and base slab 42 of the spent fuel pool 40 define acavity 43 configured to hold cooling pool water W and a plurality ofsubmerged nuclear spent fuel assembly storage racks 27 holding fuelbundles or assemblies 28 each containing multiple individual nuclearspent fuel rods. The storage racks 27 are disposed on the base slab 42in typical fashion. With continuing reference to FIGS. 1-6, the spentfuel pool 40 extends from an operating deck 22 surrounding the spentfuel pool 40 downwards to a sufficient depth D1 to submerge the fuelassemblies 28 (see, e.g. FIG. 6) beneath the surface level S of the poolwater W for proper radiation shielding purposes. In one implementation,the fuel pool may have a depth such that at least 10 feet of water ispresent above the top of the fuel assembly.

A nuclear fuel assembly storage rack 27 is shown in FIGS. 2 and 3, andfurther described in commonly assigned U.S. patent application Ser. No.14/367,705 filed Jun. 20, 1014, which is incorporated herein byreference in its entirety. The storage rack 27 contains a plurality ofvertically elongated individual cells as shown each configured forholding a fuel assembly 28 comprising a plurality of individual nuclearfuel rods. An elongated fuel assembly 28 is shown in FIG. 6 holdingmultiple fuel rods 28 a and further described in commonly assigned U.S.patent application Ser. No. 14/413,807 filed Jul. 9, 2013, which isincorporated herein by reference in its entirety. Typical fuelassemblies 28 for a pressurized water reactor (PWR) may each hold over150 fuel rods in 10×10 to 17×17 fuel rod grid arrays per assembly. Theassemblies may typically be on the order of approximately 14 feet highweighing about 1400-1500 pounds each.

The substantially horizontal operating deck 22 that circumscribes thesidewalls 41 and pool 40 on all sides in one embodiment may be formed ofsteel and/or reinforced concrete. The surface level of pool water W(i.e. liquid coolant) in the pool 40 may be spaced below the operatingdeck 22 by a sufficient amount to prevent spillage onto the deck duringfuel assembly loading or unloading operations and to account to seismicevent. In one non-limiting embodiment, for example, the surface of theoperating deck 22 may be at least 5 feet above the maximum 100 yearflood level for the site in one embodiment. The spent fuel pool 40extending below the operating deck level may be approximately 40 feet ormore deep (e.g. 42 feet in one embodiment). The fuel pool is long enoughto accommodate as many spent fuel assemblies as required. In oneembodiment, the fuel pool 40 may be about 60 feet wide. There issufficient operating deck space around the pool to provide space for thework crew and for staging necessary tools and equipment for thefacility's maintenance. There may be no penetrations in the spent fuelpool 40 within the bottom 30 feet of depth to prevent accidentaldraining of water and uncovering of the spent fuel.

According to one aspect of the invention, a spent fuel pool liner systemcomprising a double liner is provided to minimize the risk of pool waterleakage to the environment. The liner system is further designed toaccommodate cooling water leakage collection and detection/monitoring toindicate a leakage condition caused by a breach in the integrity of theliner system.

The liner system comprises a first outer liner 60 separated from asecond inner liner 61 by an interstitial space 62 formed between theliners. An outside surface of liner 60 is disposed against or at leastproximate to the inner surface 63 of the fuel pool sidewalls 41 andopposing inside surface is disposed proximate to the interstitial space62 and outside surface of liner 61. The inside surface of liner 61 iscontacted and wetted by the fuel pool water W. It bears noting thatplacement of liner 60 against liner 61 without spacers therebetweenprovides a natural interstitial space of sufficient width to allow thespace and any pool leakage there-into to be evacuated by a vacuumsystem, as further described herein. The natural surface roughness ofthe materials used to construct the liners and slight variations inflatness provides the needed space or gap between the liners. In otherembodiments contemplated, however, metallic or non-metallic spacers maybe provided which are distributed in the interstitial space 62 betweenthe liners if desired.

The liners 60, 61 may be made of any suitable metal which is preferablyresistant to corrosion, including without limitation stainless steel,aluminum, or other. In some embodiments, each liner may be comprised ofmultiple substantially flat metal plates which are seal welded togetheralong their peripheral edges to form a continuous liner systemencapsulating the sidewalls 41 and base slab 42 of the spent fuel pool40.

The inner and outer liners 61, 60 may have the same or differentthicknesses (measured horizontally or vertically between major opposingsurfaces of the liners depending on the position of the liners). In oneembodiment, the thicknesses may be the same. In some instances, however,it may be preferable that the inner liner 61 be thicker than the outerliner 60 for potential impact resistant when initially loading emptyfuel storage racks 27 into the spent fuel pool 40.

The outer and inner liners 60, 61 (with interstitial space therebetween)extend along the vertical sidewalls 41 of the spent fuel pool 40 andcompletely across the horizontal base slab 42 in one embodiment tocompletely cover the wetted surface area of the pool. This formshorizontal sections 60 b, 61 b and vertical sections 60 a, 61 a of theliners 60, 61 to provide an impervious barrier to out-leakage of poolwater W from spent fuel pool 40. The horizontal sections of liners 60 b,61 b on the base slab 42 may be joined to the vertical sections 60 a, 61a along the sidewalls 41 of the pool 40 by welding. The detail in FIG. 4shows one or many possible constructions of the bottom liner joint 64comprising the use of seal welds 65 (e.g. illustrated fillet welds orother) to seal sections 60 a to 60 b along their respective terminaledges and sections 61 a to 61 b along their respective terminal edges asshown. Preferably, the joint 64 is configured and arranged to fluidlyconnect the horizontal interstitial space 64 between horizontal linersections 60 b, 61 b to the vertical interstitial space 64 betweenvertical liner sections 60 a, 61 a for reasons explained elsewhereherein.

The top liner joint 65 in one non-limiting embodiment between the topterminal edges 60 c, 61 c of the vertical liner sections 60 a, 61 a isshown in the detail of FIG. 5. The top of the spent fuel pool 40 isequipped with a substantially thick metal embedment plate 70 whichcircumscribes the entire perimeter of the fuel pool. The embedment plate70 may be continuous in one embodiment and extends horizontally alongthe entire inner surface 63 of the sidewalls 41 at the top portion ofthe sidewalls. The embedment plate 70 has an exposed portion of theinner vertical side facing the pool which extends above the top terminalends 60 c, 61 c of the inner and outer liners 60, 61. The opposing outervertical side of the plate 70 is embedded entirely into the sidewalls41. A top surface 71 of the embedment plate 70 that faces upwards may besubstantially flush with the top surface 44 of the sidewalls 41 to forma smooth transition therebetween. In other possible implementations, thetop surface 71 may extend above the top surface 44 of the sidewalls. Theembedment plate 70 extends horizontal outward from the fuel pool 40 fora distance into and less than the lateral width of the sidewalls 41 asshown.

The embedment plate 70 has a horizontal thickness greater than thehorizontal thickness of the inner liner 61, outer liner 60, and in someembodiments both the inner and outer liners combined.

The top embedment plate 70 is embedded into the top surface 44 of theconcrete sidewalls 41 has a sufficient vertical depth or height to allowthe top terminal edges 60 c, 61 c of liners 60, 61 (i.e. sections 60 aand 61 a respectively) to be permanently joined to the plate. The topterminal edges of liners 60, 61 terminate at distances D2 and D1respectively below a top surface 71 of the embedment plate 70 (which inone embodiment may be flush with the top surface of the pool sidewalls41 as shown). Distance D1 is less than D2 such that the outer liner 60is vertical shorter in height than the inner liner 61. In oneembodiment, the embedment plate 70 has a bottom end which terminatesbelow the top terminal edges 60 c, 61 c of the liners 60, 61 tofacilitate for welding the liners to the plate.

In various embodiments, the embedment plate 70 may be formed of asuitable corrosion resistant metal such as stainless steel, aluminum, oranother metal which preferably is compatible for welding to the metalused to construct the outer and inner pool liners 60, 61 withoutrequiring dissimilar metal welding.

As best shown in FIG. 5, the top terminal edges 60 c, 61 c of inner andouter liners 60, 61 may have a vertically staggered arranged and beseparately seal welded to the top embedment plate 70 independently ofeach other. A seal weld 66 couples the top terminal edge 61 c of liner61 to the exposed portion of the inner vertical side of the embedmentplate 70. A second seal weld 67 couples the top terminal edge 60 c ofliner 60 also to the exposed portion of the inner vertical side of theembedment plate 70 at a location below and spaced vertical apart fromseal weld 66. This defines a completely and hermetically sealed enclosedflow plenum 68 that horizontal circumscribes the entire perimeter of thespent fuel pool 40 in one embodiment. The flow plenum 68 is in fluidcommunication with the interstitial space 62 as shown. One vertical sideof the flow plenum is bounded by a portion of inner liner 61 and theopposing vertical side of the plenum is bounded by the inner verticalside of the top embedment plate 70.

The top flow plenum 68 may be continuous or discontinuous in someembodiments. Where discontinuous, it is preferable that a flowpassageway 105 in the top embedment plate 70 be provided for eachsection of the separate passageways.

Seal welds 66 and 67 may be any type of suitable weld needed to seal theliners 60, 61 to the top embedment plate 70. Backer plates, bars, orother similar welding accessories may be used to make the welds asneeded depending on the configuration and dimensions of the welds used.The invention is not limited by the type of weld.

In one embodiment, the outer and inner liners 60, 61 are sealablyattached to the spent fuel pool 40 only at top embedment plate 70. Theremaining portions of the liners below the embedment plate may be inabutting contact with the sidewalls 41 and base slab 42 without meansfor fixing the liners to these portions.

It bears noting that at least the inner liner 61 has a height whichpreferably is higher than the anticipated highest water level (surfaceS) of the pool water W in one embodiment. If the water level happens toexceed that for some reason, the top embedment plate 70 will be wetteddirectly by the pool water and contain the fluid to prevent overflowingthe pool onto the operating deck 22.

According to another aspect of the invention, a vapor extraction orvacuum system 100 is provided that is used to draw down the air pressurein the interstitial space between the outer and inner liners 60, 61 to arelatively high state of vacuum for leakage control and/or detection.FIG. 7 is a schematic diagram of one embodiment of a vacuum system 100.

Referring to FIGS. 5 and 7, vacuum system 100 generally includes avacuum pump 101 and a charcoal filter 102. Vacuum pump 101 may be anysuitable commercially-available electric-driven vacuum pump capable ofcreating a vacuum or negative pressure within the interstitial space 62between the pool liners 60 and 61. The vacuum pump 101 is fluidlyconnected to the interstitial space 68 via a suitable flow conduit 103which is fluidly coupled to a telltale or flow passageway 105 extendingfrom the top surface 71 of the top embedment plate 70 to the top flowplenum 68 formed between the pool liners 60 and 61. Flow conduit 103 maybe formed of any suitable metallic or non-metallic tubing or pipingcapable of withstanding a vacuum. A suitably-configured fluid coupling104 may be provided and sealed to the outlet end of the flow passageway105 for connecting the flow conduit 103. The inlet end of the flowpassageway penetrates the inner vertical side of top embedment plate 70within the flow plenum 68. The flow passageway 105 and external flowconduit 103 provides a contiguous flow conduit that fluidly couples theflow plenum 68 to the vacuum pump 101. A one-way check valve is disposedbetween the flow plenum 105 and the suction inlet of the vacuum pump 101to permit air and/or vapor to flow in a single direction from the linersystem to the pump.

The absolute pressure maintained by the vacuum system 100 in theinterstitial space 62 between the liners 60, 61 (i.e. “set pressure”)preferably should be such that the bulk water temperature of the spentfuel pool 40 which is heated by waste decay heat generated from the fuelrods/assemblies is above the boiling temperature of water at the setpressure. The table below provides the boiling temperature of water atthe level of vacuum in inches of mercury (Hg) which represent someexamples of set pressures that may be used.

Pressure in inch, HgA Boiling Temp, deg F. 1 79 2 101 3 115 4 125 5 133

Any significant rise in pressure would indicate potential leakage ofwater in the interstitial space 62 between the liners 60, 61. Because ofsub-atmospheric conditions maintained by the vacuum pump 101 in theinterstitial space, any water that may leak from the pool into thisspace through the inner liner 61 would evaporate, causing the pressureto rise which may be monitored and detected by a pressure sensor 104.The vacuum pump 101 preferably should be set to run and drive down thepressure in the interstitial space 62 to the “set pressure.”

In operation as one non-limiting example, if the vacuum pump 101 isoperated to create a negative pressure (vacuum) in the interstitialspace 62 of 2 inches of Hg, the corresponding boiling point of water atthat negative pressure is 101 degrees Fahrenheit (degrees F.) from theabove Table. If the bulk water temperature of pool water W in the spentfuel pool 40 were at any temperature above 101 degrees F. and leakageoccurred through the inner pool liner 61 into the interstitial space 62,the liquid leakage would immediately evaporate therein creating steam orvapor. The vacuum pump 101 withdraws the vapor through the flow plenum68, flow passageway 105 in the top embedment plate 70, and flow conduit103 (see, e.g. directional flow arrows of the water vapor in FIGS. 5 and7). Pressure sensor 104 disposed on the suction side of the pump 101would detect a corresponding rise in pressure indicative of a potentialleak in the liner system. In some embodiments, the pressure sensor 104may be operably linked to a control panel of a properly configuredcomputer processor based plant monitoring system 107 which monitors anddetects the pressure measured in the interstitial space 62 between theliners on a continuous or intermittent basis to alert operators of apotential pool leakage condition. Such plant monitoring systems are wellknown in the art without further elaboration.

The extracted vapor in the exhaust or discharge from the vacuum pump 101is routed through a suitable filtration device 102 such as a charcoalfilter or other type of filter media before discharge to the atmosphere,thereby preventing release of any particulate contaminants to theenvironment.

Advantageously, it bears noting that if leakage is detected from thespent fuel pool 40 via the vacuum system 100, the second outer liner 60encapsulating the fuel pool provides a secondary barrier and line ofdefense to prevent direct leaking of pool water W into the environment.

It bears noting that there is no limit to the number of vapor extractionsystems including a telltale passageway, vacuum pump, and filtercombination with leakage monitoring/detection capabilities that may beprovided. In some instances, four independent systems may provideadequate redundancy. In addition, it is also recognized that a third oreven fourth layer of liner may be added to increase the number ofbarriers against leakage of pool water to the environment. A third layerin some instances may be used as a palliative measure if the leaktightness of the first inter-liner space could not, for whatever reason,be demonstrated by a high fidelity examination in the field such ashelium spectroscopy.

While the foregoing description and drawings represent exemplaryembodiments of the present disclosure, it will be understood thatvarious additions, modifications and substitutions may be made thereinwithout departing from the spirit and scope and range of equivalents ofthe accompanying claims. In particular, it will be clear to thoseskilled in the art that the present invention may be embodied in otherforms, structures, arrangements, proportions, sizes, and with otherelements, materials, and components, without departing from the spiritor essential characteristics thereof. In addition, numerous variationsin the methods/processes described herein may be made within the scopeof the present disclosure. One skilled in the art will furtherappreciate that the embodiments may be used with many modifications ofstructure, arrangement, proportions, sizes, materials, and componentsand otherwise, used in the practice of the disclosure, which areparticularly adapted to specific environments and operative requirementswithout departing from the principles described herein. The presentlydisclosed embodiments are therefore to be considered in all respects asillustrative and not restrictive. The appended claims should beconstrued broadly, to include other variants and embodiments of thedisclosure, which may be made by those skilled in the art withoutdeparting from the scope and range of equivalents.

What is claimed is:
 1. A spent fuel pool system comprising: a fuel poolcomprising: a base slab; a plurality of vertical sidewalls extendingfrom the base slab; and a cavity defined by the vertical sidewalls andthe base slab, the cavity being at least partially filled with poolwater; a pool liner system located within the cavity, the pool linersystem comprising: an outer liner positioned adjacent to the verticalsidewalls; an inner liner positioned adjacent to the outer liner suchthat the inner liner is in contact with the pool water; and wherein theinner and outer liners are separated by an air gap.
 2. The spent fuelpool system according to claim 1 wherein both of the inner and outerliners are positioned within the cavity of the fuel pool.
 3. The spentfuel pool system according to claim 1 further comprising an embedmentplate embedded into the vertical sidewalls and circumscribing thecavity, the embedment plate comprising an inner surface that faces thecavity.
 4. The spent fuel pool system according to claim 3 wherein theinner liner is sealably coupled to the inner surface of the embedmentplate at a first position and the outer liner is sealably coupled to theinner surface of the embedment plate at a second position, the firstposition being closer to an open top end of the cavity than the secondposition.
 5. The spent fuel pool system according to claim 4 wherein ahermetically sealed flow plenum is formed between a top terminal edge ofthe inner liner and a top terminal edge of the outer liner, wherein theflow plenum is in fluid communication with the air gap.
 6. The spentfuel pool system according to claim 5 further comprising a flowpassageway formed through the embedment plate, a first end of the flowpassageway being fluidly coupled to the flow plenum.
 7. The spent fuelpool system according to claim 6 further comprising a vacuum systemlocated outside of the cavity of the fuel pool, the vacuum systemcomprising a vacuum pump that is fluidly coupled to the air gap by wayof the flow passageway of the embedment plate.
 8. The spent fuel poolsystem according to claim 7 wherein the vacuum pump is configured tomaintain the air gap at a vacuum pressure.
 9. The spent fuel pool systemaccording to claim 8 further comprising a pressure sensor configured todetect any rise in pressure within the air gap that would be indicativeof a potential leak in the pool liner system.
 10. The spent fuel poolsystem according to claim 5 wherein the flow plenum extends around anentire perimeter of the spent fuel pool.
 11. The spent fuel pool systemaccording to claim 1 wherein the air gap extends interrupted from aninner surface of the outer liner that faces away from the verticalsidewalls and towards the cavity to an outer surface of the inner linerthat faces away from the cavity and towards the vertical sidewalls. 12.The spent fuel pool system according to claim 1 further comprising aplurality of storage racks disposed on the base slab and submerged inthe pool water, each of the storage racks having a plurality of cellseach configured for holding a spent nuclear fuel assembly containingnuclear fuel rods.
 13. The spent fuel pool system according to claim 1wherein the inner and outer liners are located entirely within thecavity of the fuel pool.
 14. A spent fuel pool system comprising: a fuelpool comprising: a base slab; a plurality of vertical sidewallsextending from the base slab; and a cavity defined by the base slab andan inner surface of the vertical sidewalls, the cavity being filled withpool water; a pool liner system comprising: an outer liner positionedadjacent to the inner surfaces of the vertical sidewalls; and an innerliner positioned adjacent to the outer liner, the inner liner beingspaced apart from the outer liner by an air gap, and wherein the innerliner is in contact with the pool water; and a vacuum system fluidlycoupled to the air gap, the vacuum system comprising a vacuum pump thatis configured to maintain the air gap at a vacuum pressure.
 15. Thespent fuel pool system according to claim 14 further comprising apressure sensor configured to detect any rise in pressure within the airgap that would be indicative of a potential leak in the pool linersystem.
 16. The spent fuel pool system according to claim 14 furthercomprising a flow plenum formed between a top terminal edge of the outerliner and a top terminal edge of the inner liner, the flow plenum beingfluidly coupled to the air gap between the inner and outer liners. 17.The spent fuel pool system according to claim 16 wherein the vacuum pumpis fluidly coupled to the air plenum through a flow passageway formedinto an embedment plate that is embedded within the vertical sidewalls.18. A spent fuel pool system comprising: a base slab; a plurality ofvertical sidewalls extending from the base slab; a cavity defined by thevertical sidewalls and the base slab, the cavity being filled with poolwater; at least one fuel storage rack disposed in the cavity, the fuelstorage rack holding a nuclear spent fuel assembly containing nuclearfuel rods that heat the pool water; a pool liner system located in thecavity and comprising an outer liner adjacent the sidewalls and baseslab, an inner liner adjacent the outer liner and wetted by the poolwater, and an interstitial space formed between the outer and innerliners; and a vacuum pump fluidly coupled to the interstitial space, thevacuum pump being configured to evacuate air from the interstitial spaceto maintain a pressure of the interstitial space below atmosphericpressure.
 19. The spent fuel pool system according to claim 18 furthercomprising a pressure sensor configured to detect any rise in pressurewithin the air gap that would be indicative of a potential leak in thepool liner system.
 20. The spent fuel pool system according to claim 19,further comprising a computer processor based plant monitoring systemoperably coupled to the pressure sensor to monitor the pressure of theinterstitial space.